In many applications, particularly in image or video retrieval from large datasets, classifiers are commonly used to assign a score indicating the probability that a given item (image or video) belongs to a particular category. In one application, classifiers are used to retrieve video clips of a particular category, such as to automate a task otherwise performed by an intelligent analyst. Commercial classifiers can be used to detect abnormal behavior in airports or shoplifting in retail settings.
Classifiers are generally trained to assess probability given a particular feature of the input image or input video. The features are chosen by system designers to exploit a cue to the underlying category. In systems that distinguish between similar categories, more than one cue, and thus more than one type of feature may be used.